Saturday 7 April 2012

Planets discovered orbiting the ancient, second generation star HIP 11952.

The Universe, cosmologists currently theorize, is about 13.75 billion years old. During this time it is thought to have produced three generations of stars, known as Populations I, II and III. The first of these generations (paradoxically known as Population III) is thought to have formed not long after the Big Bang (origin of the Universe), possibly within 300 million years.

Population III stars are thought to have been supermassive objects (up to 130 times the mass of the sun) comprised entirely of Hydrogen and Helium, the only elements produced in the cooling early universe. Such supermassive, metal-free, stars would have had short lives, at most a few million years, then undergone massive supernova type explosions, producing the next 24 elements of the periodic table, Lithium-Iron. Interestingly, no Population III star has ever been directly detected, one of the biggest problems for our current understanding of the Universe's origins.

Population III stars formed from the gas-clouds of the cooling of the early Universe. The fed on this gas, swelling to sizes in excess of 100 times that of our Sun. David Aguilar/Harvard-Smithsonian Center for Astrophysics.

Population II stars are thought to have formed from the remains of Population III stars. They are considered to be metal poor, but do contain small amounts of all the first 26 elements (this is a relative term, all stars are comprised almost entirely of Hydrogen and Helium). All naturally occurring elements heavier than Iron (except those that are decay products of heavier elements) are thought to have been formed in supernovas at the end of the lives of Population II stars. Despite this such stars are still fairly common.

Population I stars are the most common stars in today's Universe; our own Sun is a Population II star, as are most of the stars in our close vicinity. They are considered 'metal-rich' (again a relative term, they are comprised of almost entirely of Hydrogen and Helium) and contain trace amounts of all the elements of the periodic table.

To date almost all exoplanets that have been discovered to date have orbited Population I stars, however there is a distinct possibility that this is due to sampling error, as these are the most common stars in the Universe, and we inhabit a stellar neighborhood in which they are particularly abundant. With this in mind astronomers have started to specifically look for planets orbiting Population II stars.

In a forthcoming paper in the journal Astronomy & Astrophysics, a team of scientists led by Johny Setiawan of the Max-Planck-Institut für Astronomie describe the discovery of a pair of planets orbiting the Population II star HIP 11952, 376 light years from Earth, using the 2.2 m Max-Planck Gesellschaft/European Southern Observatory (MPG/ESO) telescope at the ESO La Silla Observatory.

Setiawan et al. measure HIP 11952 as having 0.83 times the mass of our Sun, and 17.2 times the volume; it is an F-type star with an effective temperature of 6120 K (compared to 5778 K for our Sun). The volume of a gas is derived from the number of molecules/atoms present and the temperature, not the mass. For example Oxygen molecules (O₂) are 16 times as massive as Hydrogen molecules (H₂), so one kilogram of O₂ will occupy the same volume as sixteen kilograms of Hydrogen at the same temperature, since it contains the same number of molecules. Increase the temperature and the volume increases. Thus the hotter and more metal-poor a star is, the greater its volume.

The researchers detected two planets orbiting this star, HIP 11592b, a planet with a mass of 2.93 times that of Jupiter orbiting every 290 days at 81% of the distance at which the Earth orbits the sun and HIP 11592c, a planet with a mass 0.78 times that of Jupiter, orbiting every 6.95 days at a distance of 7% the distance between the Earth and the Sun.

An artist's impression of the planets of the HIP 11952 system. Timotheos Samartzidis.

HIP 11952 is thought to be 12.8 billion years old (i.e. 93% of the age of the Universe), so if the planets are the same are as the star they orbit (which is the usual assumption) then these would be the oldest planets yet discovered, which is how they have widely been reported in the press. However there is a problem with this. Planets close to stars tend to lose mass due to the radiation from the star. This is particularly true if the star is hot, or the planets are composed largely of lighter elements, both of which apply in the HIP 11952 system. Thus for the planets, particularly the inner one, to have survived this long, then they would have to have been truly immense to start with.

This makes an alternative scenario rather more likely (but not certain). There are three obvious possibilities for this. Firstly the age currently attributed to the HIP 11952 system could be wrong. This is not unthinkable, but would have serious implications as if our methods of dating stars are wrong, then this is probably not the only star with the wrong age attributed to it. Secondly the planets could have originated further out in the system and have recently (comparatively recently, not necessarily within the last billion years) moved inwards within the system. Thirdly the planets could have originated outside the HIP 11952 system and have been captured by the star at some point. Thus the planets could be considerably less than 12.5 billion years old, have a composition containing more heavy elements, and would not have been exposed to the heat of the star for so long.

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